The Antigua-based accountancy firm responsible for ensuring Sir Allen Stanford's business empire was run on a proper financial footing was yesterday unwilling - or unable - to explain how it appeared to have missed the alleged fraud.

CAS Hewlett, auditor for Stanford International Bank (SIB), "is as surprised as anybody else" by claims the group was involved in massive financial misconduct dating back to the mid-1990s, said Eugene, an employee in Antigua, who declined to give details about the company and its work with SIB, or even his full name.

The company, he said, was in a state of transition while London-based Celia Hewlett takes control of the company previously managed by her father, Charlesworth Hewlett, who died in January. Eugene said he could not give any contact information for Hewlett.

CAS Hewlett had a "team" working on the SIB account, although there were matters that only Charlesworth Hewlett had dealt with, Eugene said. "I have no idea how he missed [any possible fraud]," he added.

Willis Ltd, a London-based insurance broker handling risk management and reinsurance that is listed in SIB documents as the bank's insurance and risk manager, also declined to comment.

While Willis is a major international firm with offices around the globe, CAS Hewlett is a more local organisation, and an unusual choice to audit an international bank with $8bn in assets.

A more traditional choice for a bank the size of SIB, with offices in Texas, London and Canada, would be one of the major international accountancy firms such as KPMG or PricewaterhouseCoopers with expertise in crossborder investment, corporate finance and tax issues.

"The auditor is a local firm," Alex Dalmady, a Florida-based analyst who raised questions about SIB in January wrote in a magazine article at the time. "The principal is a 72-year old gentleman who has been auditing the bank for many years, at least 10. PWC and KPMG have offices on the island, and it's a good internal control practice to change auditors every few years, but the bank prefers to stick with its trusted firm." Accountants have pointed out that Madoff, the US fund involved in an alleged $50m fraud, was also audited by a small firm.